Laser processing apparatus and method

ABSTRACT

A controller outputs a first event signal having a periodical waveform and a second event signal having a periodical waveform synchronizing with the first event signal. A first laser source radiates a first pulse laser beam having a wavelength in an ultraviolet range, synchronously with the first event signal. A second laser source radiates a second pulse laser beam having a wavelength in the ultraviolet range, synchronously with the second event signal. A converging optical system converges the first and second pulse laser beams at the same point. A holder holds a workpiece at a position where a pulse laser beam converged by the converging optical system is applied.

[0001] This application is based on Japanese Patent Application2000-173244, filed on Jun. 9, 2000, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] a) Field of the Invention

[0003] The present invention relates to a laser processing apparatus andmethod, and more particularly to a laser processing apparatus and methodfor applying a pulse laser beam having a wavelength in an ultravioletrange to a workpiece and forming a hole in or through the workpiece.

[0004] b) Description of the Related Art

[0005] A conventional laser processing method will be described bytaking as an example a method of forming a hole in or through amulti-layer wiring substrate. An infrared pulse laser beam radiated froma carbon dioxide gas laser oscillator is converged at a resin layer of amulti-layer wiring substrate. Organic substance in the region appliedwith the laser beam is thermally decomposed and a hole is formed in thisregion. With this method, a through hole 100 to 200 μm in diameter canbe formed through a resin layer about 40 to 80 μm thick. A carbondioxide gas laser oscillator can radiate a pulse laser beam having ahigh energy per one pulse. This pulse laser beam can form a throughhole, for example, by three shots.

[0006] Holes having shorter diameters are desired to be formed in amulti-layer wiring substrate of a semiconductor integrated circuitdevice which is implemented at a higher integration density. A lowerlimit of the diameter of a hole is about five times the wavelength of alaser beam used. If a carbon dioxide laser is used, the lower limit of ahole is about 50 μm. It is practically difficult to form a hole having adiameter smaller than 50 to 60 μm by using a carbon dioxide gas laser.

[0007] If a laser beam having a wavelength in the ultraviolet range isused, a hole having a smaller diameter can be formed. It is difficult,however, to generate a laser beam having a wavelength in the ultravioletrange and a large power. If a laser beam having a small power is usedfor processing a multi-layer wiring substrate, a process time prolongsand productivity lowers.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide a laserprocessing apparatus and method capable of shortening a process time byusing a laser beam having a wavelength in the ultraviolet range.

[0009] According to one aspect of the present invention, there isprovided a laser processing apparatus comprising: controller foroutputting a first event signal having a periodical waveform and asecond event signal having a periodical waveform synchronizing with thefirst event signal; a first laser source for radiating a first pulselaser beam having a wavelength in an ultraviolet range, synchronouslywith the first event signal; a second laser source for radiating asecond pulse laser beam having a wavelength in the ultraviolet range,synchronously with the second event signal; a converging optical systemfor converging the first and second pulse laser beams at a same point;and holder for holding a workpiece at a position where a pulse laserbeam converged by the converging optical system is applied.

[0010] According to another aspect of the present invention, there isprovided a laser processing method comprising the steps of: radiating afirst pulse laser beam from a first laser source, the first pulse laserbeam having a wavelength in an ultraviolet range; radiating a secondpulse laser beam from a second laser source synchronously with the firstpulse laser beam, the second pulse laser beam having a wavelength in theultraviolet range; and applying the first and second pulse laser beamsto a same processing area of a workpiece to form a hole in the sameprocessing area.

[0011] When the pulses of the first and second pulse laser beams arealternately applied to the same point of a workpiece, a process speedcan be approximately doubled. When the pulses of the first and secondpulse laser beams are overlapped, the energy per one pulse can beincreased so that a workpiece can be processed which requires a largeenergy for forming a hole.

[0012] According to another aspect of the present invention, there isprovided a laser processing method comprising the steps of: preparing aworkpiece having a first layer and a second layer formed under the firstlayer, wherein a hole can be formed in the first layer by applying anultraviolet pulse laser beam having a first energy per one pulse, and ahole can be formed in the second layer by applying an ultraviolet pulselaser beam having not the first energy per one pulse but a second energyper one pulse higher than the first energy; applying a first pulse laserbeam and a second pulse laser beam to the first layer in a processingarea thereof under a timing condition that pulses of the first andsecond pulse laser beams are alternately applied to the first layer, toform a first hole in the first layer and expose a partial surface of thesecond layer under the first layer, the first pulse laser beam beingradiated from a first laser source and having a wavelength in anultraviolet range, and the second pulse laser beam being radiated from asecond laser source and having a wavelength in the ultraviolet range;and applying the first and second pulse laser beams to the second layerexposed on a bottom of the first hole under a timing condition thatpulses of the first and second pulse laser beams are at least partiallyoverlapped, to form a second hole in the second layer, the first pulselaser beam being radiated from the first laser source and having thewavelength in the ultraviolet range, and the second pulse laser beambeing radiated from the second laser source and having the wavelength inthe ultraviolet range.

[0013] By chanting the timing conditions of the first and second pulselaser beams, the first and second layers can be processed continuously.

[0014] According to another aspect of the present invention, there isprovided a laser processing apparatus comprising: controller foroutputting a first event signal having a periodical waveform and asecond event signal having a periodical waveform synchronizing with thefirst event signal; a first laser source for radiating a first pulselaser beam having a wavelength in an infrared or visual range,synchronously with the first event signal; a second laser source forradiating a second pulse laser beam having a wavelength in the infraredor visual range, synchronously with the second event signal; an opticalpropagation system for changing an optical axis of at least one of thefirst and second laser beams so as to make the first and second pulselaser beams propagate along a same optical axis; a non-linear opticalcomponent for generating a harmonic wave having a wavelength in anultraviolet range, from the first and second pulse laser beams made tohave the same optical axis by the optical propagation system; aconverging optical system for converging the harmonic wave; and holderfor holding a workpiece at a position where the harmonic wave convergedby the converging optical system is applied.

[0015] As the pulses of the first and second pulse laser beamsalternately reach the non-linear optical component, a harmonic wave isgenerated having a repetition frequency twice as high as the repetitionfrequency of each of the input pulse laser beams. A process time cantherefore be shortened. When the pulses of the first and second pulselaser beams are overlapped and become incident upon the non-linearoptical component, the energy per one pulse of the harmonic waveincreases so that a workpiece can be processed which requires a largeenergy for forming a hole.

[0016] As above, by combining the pulse laser beams radiated from thetwo laser sources to have a predetermined phase difference, the holeforming time can be shortened. By overlapping the pulses of the firstand second pulse laser beams, the energy per one pulse can be increased.Even if a sufficient energy per one pulse cannot be obtained by onelaser source, a sufficient energy can be obtained by using two lasersources. A hole can be formed in a workpiece even if it requires a largeenergy per one pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a block diagram showing a laser processing apparatusaccording to an embodiment of the invention.

[0018]FIG. 2 is a timing chart showing the operation of a first controlmode of the laser processing apparatus according to the embodiment.

[0019]FIG. 3 is a timing chart showing the operation of a second controlmode of the laser processing apparatus according to the embodiment.

[0020]FIG. 4 is a graph showing an example of the output characteristicsof a third harmonic wave of an Nd : YAG laser.

[0021]FIG. 5 is a cross sectional view of a multi-layer wiringsubstrate.

[0022]FIG. 6 is a block diagram showing a laser processing apparatusaccording to another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023]FIG. 1 is a block diagram showing a laser processing apparatusaccording to an embodiment of the invention. First and second lasersources 1 and 2 radiate pulse laser beams pl₁ and pl₂ having awavelength in the ultraviolet range, synchronously with event signalssig₁ and sig₂. The first and second laser sources 1 and 2 each include,for example, an Nd : YAG laser oscillator and non-linear opticalcomponents. The pulse laser beams ply and pl₂ each are, for example, athird harmonic wave (355 nm in wavelength) of a pulse laser beamradiated from an Nd : YAG laser oscillator. The pulse laser beams pl₁and pl₂ are linearly polarized, respectively in vertical and horizontaldirections.

[0024] The pulse laser beam pl₁ radiated from the first laser source 1is reflected by a turn-around mirror 5 and becomes incident upon thefront surface of a polarizer 6 at an incidence angle of 45°. The pulselaser beam pl₂ radiated from the second laser source 2 is incident uponthe back surface of the polarizer 6 at an incidence angle of 45°. Thepolarizer 6 reflects the pulse laser beam pl₁ which was linearlypolarized in the vertical direction, and transmits the pulse laser beampl₂ which was linearly polarized in the horizontal direction.

[0025] The pulse laser beams pl₁ and pl₂ are combined on the sameoptical axis by the polarizer 6 to form a pulse laser beam pl₃. Thepulse laser beam pl₃ is reflected by a turn-around mirror 9. Thereflected pulse laser beam pl₄ becomes incident upon a galvano scanner10. The galvano scanner scans the optical axis of the pulse laser beampl₄ in a two-dimensional direction in response to a command signal sig₀.

[0026] The pulse laser beam passed through the galvano scanner 10 isconverged by a converging lens 11 to form a pulse laser beam pl₅. Forexample, the converging lens 11 is an fθ lens. A workpiece 20 is held bya holder 12 at a converging position of the pulse laser beam pl₅.

[0027] A control unit 13 supplies the first and second laser sources 1and 2 with the event signals sign and sig₂ having a periodical waveform.The control unit 13 selects one of first and second control modes andcan supply the event signals sig₁ and sig₂ having a phase differencespecific to each control mode. The control unit 13 also supplies thegalvano scanner 10 with the control signal sig₀.

[0028] Next, with reference to FIGS. 2 and 3, timings of the pulse laserbeams used by the laser processing apparatus shown in FIG. 1 will bedescribed.

[0029]FIG. 2 is a timing chart of the first control mode. The eventsignals sig₁ and sig₂ are pulse signals having the same frequency andsynchronized with each other. The phase of the event signal sig₂ lags by180 degrees from the phase of the event signal sig₁. The pulse laserbeam pl₁ is synchronous with the event signal sig₁, whereas the pulselaser beam pl₂ is synchronous with the event signal sig₂. The pulselaser beam pl₂ lags therefore by 180°0 in phase from the pulse laserbeam pl₁. The pulse repetition frequencies of the pulse laser beams pl₃to pl₅ formed through combination of the pulse laser beams pl₁ and pl₂are twice the frequency of the event signals sig, and sig₂.

[0030]FIG. 4 is a graph showing an example of the output characteristicsof a third harmonic wave of each of the first and second laser sources 1and 2 using Nd : YAG laser oscillators. The abscissa represents a pulserepetition frequency in the unit of “kHz”, and the ordinate represents alaser output in the unit of “W”. At the repetition frequency of about 5kHz, the laser output takes a maximum value. In the repetition frequencyrange not lower than 5 kHz, the laser output gradually lowers as therepetition frequency becomes high. This tendency is not limited only toan Nd : YAG laser oscillator, but other solid state lasers have similartendency.

[0031] In order to form a hole in or through a resin film, an energydensity per one pulse of a pulse laser beam is generally required tohave some threshold value or higher. For example, if a hole is to beformed in an epoxy resin film, the energy density per one pulse isrequired to have about 1 J/cm² or higher. An energy per one pulsenecessary for forming a hole is determined from the area of the hole.The energy per one pulse is given by p/f [J], where P [W] is an outputof the pulse laser beam and f [Hz] is a pulse repetition frequency. Therange where the energy P/f per one pulse takes the necessary thresholdvalue or higher can be determined from the output characteristics shownin FIG. 4. If the laser sources 1 and 2 are operated in this range, ahole can be formed in a resin film.

[0032] The repetition frequency of the pulse laser beam pl₅ applied tothe workpiece 20 is 10 kHz which is a twofold of the frequency of theevent signals sig, and sig₂. The hole forming time can be shortened byabout ½ as compared to using one laser oscillator.

[0033]FIG. 3 is a timing chart of the second control mode. In the firstcontrol mode shown in FIG. 2, the phase of the event signal sig₂ lags by180° from the phase of the event signal sig₁. In the second controlmode, the phase lag is small. Accordingly, the pulse laser beams pl₁ andpl₂ partially overlap to form the pulse laser beams pl₃ to pl₅ formedthrough combination of the pulse laser beams pl₁ and pl₂. The width ofeach pulse is hence broadened and the energy per one pulse is doubled.The phases of the event signals sig₁ and sig₂ may be set equal tocompletely superpose each pulse of the pulse laser beam pl₁ upon eachpulse of the pulse laser beam pl₂. In this case, the pulse width doesnot broaden but the peak power is approximately doubled.

[0034] In order to form a hole in a copper foil, the energy density perone pulse is generally required to be about 10 J/cm² or higher. If thediameter of a hole is 100 μm, the energy per one pulse is required to beabout 7.9×10⁻⁴ J or higher. In the first control mode shown in FIG. 2,it is difficult to set the energy per one pulse to about 7.9×10⁻⁴ J orhigher. By partially overlapping the two pulse laser beams as shown inFIG. 3, the energy per one pulse necessary for forming a hole in acopper foil can be obtained.

[0035] Even if the energy per one pulse is insufficient, the necessaryenergy per one pulse may be obtained by converging the laser pulse andreducing the beam diameter. However, in this case, since the laserdiameter is small, it is necessary to move the application position ofthe laser beam in order to form a hole having a desired size. Forexample, trepanning or spiral working becomes necessary. As in thisembodiment, by increasing the energy per one pulse, a hole having adiameter of about 100 μm can be formed without trepanning or the like.

[0036] For example, if the repetition frequency is set to 10 kHz, theoutput of one laser source is about 4 W as determined from FIG. 4. Thepower of each of the laser beams pl₃ to pl₅ shown in FIG. 3 is therefore8 W. The energy per one pulse is 8×10⁻⁴ J. Although one laser source isdifficult to form a hole in a copper foil, the energy per one pulse canbe made sufficiently large for forming a hole in a copper foil by usingtwo laser sources and superposing pulses.

[0037] The pulse width and peak intensity of the laser beams pl₃ to pl₅shown in FIG. 3 depend on the phase difference between the pulse laserbeams pl₁ and pl₂. By adjusting the phase difference between the eventsignals sig₁ and sig₂, the pulse width and peak intensity of the pulselaser beams pl₃ to pl₅ can be controlled with ease.

[0038]FIG. 5 is a cross sectional view of a multi-layer wiringsubstrate. A package board 22 is mounted on the surface of a motherboard 21. A semiconductor integrated circuit chip 23 is mounted on thepackage board 22. The mother board 21 and package board 22 are made ofepoxy resin which contains glass cloth.

[0039] Copper wiring layers 25 are formed embedded in the mother board21. A via hole 26 extends from the surface of the mother board 21 to thecopper wiring layer 25. A through hole 27 is formed through the motherboard 21. Copper is filled in the via hole 26 and through hole 27.Similarly, a copper wiring layer 28 and a via hole 29 are formed in thepackage board 22. The via holes 26 and 29 and through hole 27 are formedby using the laser processing apparatus shown in FIG. 1. The laserprocessing is executed for separate mother board 21 and package board 22before the latter 22 is mounted on the former 21.

[0040] The via holes 26 and 29 are formed in the first control modeshown in FIG. 2. In this case, the energy per one pulse of the pulselaser beam pl₅ is sufficiently large for forming a hole in the resinlayer. However, since the energy is insufficient for forming a hole inthe copper wiring layer, the copper wiring layer 25 is left unetched onthe bottom of the via hole.

[0041] In order to form the through hole 27, after a hole is formedthrough the resin layer in the first control mode, another hole isformed through the copper wiring layer in the second control mode shownin FIG. 3. In this latter case, the energy per one pulse of the pulselaser beam pl₅ is sufficiently large for forming a hole in the copperfoil layer. The through hole 27 can be formed in this manner byalternately repeating the laser processing in the first and secondcontrol modes.

[0042] If a hole is to be formed through a copper foil layer formed onthe surface of a resin substrate, the hole is formed through the copperfoil layer first in the second control mode. This laser processing canbe stopped automatically when the hole is formed through the copper foillayer, by setting beforehand the number of pulses to be applied, inaccordance with the thickness of the copper foil layer. After the holeis formed through the copper foil layer, then the mode is switched tothe first control mode and another hole is formed through the resinlayer. The number of pulses applied during the laser processing in thefirst control mode is also set beforehand.

[0043] In this embodiment, a third harmonic wave of an Nd : YAG laser isused as the pulse laser beam having a wavelength in the ultravioletrange. Other laser beams may also be used. For example, a fourth orfifth harmonic wave of an Nd : YAG laser may be used, and a YLF laser orYVO₄ laser may be used instead of an Nd : YAG laser. A fundamental waveof a KrF excimer laser or XeCl excimer laser may also be used.

[0044] Also in this embodiment, the pulse laser beam p₁ radiated fromthe first laser source 1 and the pulse laser beam pl₂ radiated from thesecond laser source 2 are propagated along the same optical axis andconverged at a working position of a workpiece. It is not necessarilyrequired to propagate the pulse laser beams pl₁ and pl₂ along the sameoptical axis. For example, the first and second pulse laser beams pl₁and pl₂ may be propagated along different optical axes which are crossedat the working position of a workpiece.

[0045] Next, with reference to FIG. 6, another embodiment of theinvention will be described. In the first embodiment, third harmonicwaves of the two Nd : YAG lasers are combined, whereas in the secondembodiment, fundamental waves are combined and then the third harmonicwave is formed. The fundamental structure of a laser processingapparatus shown in FIG. 6 is similar to that of the laser processingapparatus shown in FIG. 1. Only different points between the twoapparatus will be described.

[0046] As shown in FIG. 6, first and second laser sources 1 and 2radiate pulse laser beams pl₁ and pl₂having a wavelength in the infraredor visual range. A non-linear optical component 15 is disposed on theoptical axis of a pulse laser beam pl₃ formed through combination of thetwo pulse laser beams pl₁ and pl₂. The non-linear optical component 15generates a harmonic wave, e.g., third harmonic wave, of the pulse laserbeam pl₃. The non-linear optical component 15 may be disposed anywherealong the optical path of the pulse laser beam from a polarizer 6 to aworkpiece 20.

[0047] The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It is apparent that various modifications, improvements,combinations, and the like can be made by those skilled in the art.

What is claimed is:
 1. A laser processing apparatus comprising:controller for outputting a first event signal having a periodicalwaveform and a second event signal having a periodical waveformsynchronizing with the first event signal; a first laser source forradiating a first pulse laser beam having a wavelength in an ultravioletrange, synchronously with the first event signal; a second laser sourcefor radiating a second pulse laser beam having a wavelength in theultraviolet range, synchronously with the second event signal; aconverging optical system for converging the first and second pulselaser beams at a same point; and holder for holding a workpiece at aposition where a pulse laser beam converged by said converging opticalsystem is applied.
 2. A laser processing apparatus according to claim 1, wherein said converging optical system changes at least one of thefirst and second pulse laser beams and converges the first and secondpulse laser beams in such a manner that the first and second pulse laserbeams are propagated along a same optical axis.
 3. A laser processingapparatus according to claim 1 , wherein said controller provides afirst control mode and a second control mode, outputs the first andsecond event signals in the first control mode in such a manner thatpulses of the first and second pulse laser beams are alternately appliedto a processing position of the workpiece, and outputs the first andsecond event signals in the second control mode in such a manner thatpulses of the first and second pulse laser beams are at least partiallyoverlapped and applied to the processing position.
 4. A laser processingapparatus according to claim 3 , wherein said controller can control inthe second control mode an overlap degree between the pulses of thefirst and second pulse laser beams.
 5. A laser processing methodcomprising the steps of: radiating a first pulse laser beam from a firstlaser source, the first pulse laser beam having a wavelength in anultraviolet range; radiating a second pulse laser beam from a secondlaser source synchronously with the first pulse laser beam, the secondpulse laser beam having a wavelength in the ultraviolet range; andapplying the first and second pulse laser beams to a same processingarea of a workpiece to form a hole in the same processing area.
 6. Alaser processing method according to claim 5 , wherein in said step offorming a hole, a synchronous state between the first and second pulselaser beams is controlled in such a manner that pulses of the first andsecond pulse laser beams are alternately applied to the workpiece.
 7. Alaser processing method according to claim 5 , wherein in said step offorming a hole, a synchronous state between the first and second pulselaser beams is controlled in such a manner that pulses of the first andsecond pulse laser beams are at least partially overlapped in the sameprocessing area.
 8. A laser processing method comprising the steps of:preparing a workpiece having a first layer and a second layer formedunder the first layer, wherein a hole can be formed in the first layerby applying an ultraviolet pulse laser beam having a first energy perone pulse, and a hole can be formed in the second layer by applying anultraviolet pulse laser beam having not the first energy per one pulsebut a second energy per one pulse higher than the first energy; applyinga first pulse laser beam and a second pulse laser beam to the firstlayer in a processing area thereof under a timing condition that pulsesof the first and second pulse laser beams are alternately applied to thefirst layer, to form a first hole in the first layer and expose apartial surface of the second layer under the first layer, the firstpulse laser beam being radiated from a first laser source and having awavelength in an ultraviolet range, and the second pulse laser beambeing radiated from a second laser source and having a wavelength in theultraviolet range; and applying the first and second pulse laser beamsto the second layer exposed on a bottom of the first hole under a timingcondition that pulses of the first and second pulse laser beams are atleast partially overlapped, to form a second hole in the second layer,the first pulse laser beam being radiated from the first laser sourceand having the wavelength in the ultraviolet range, and the second pulselaser beam being radiated from the second laser source and having thewavelength in the ultraviolet range.
 9. A laser processing apparatuscomprising: controller for outputting a first event signal having aperiodical waveform and a second event signal having a periodicalwaveform synchronizing with the first event signal; a first laser sourcefor radiating a first pulse laser beam having a wavelength in aninfrared or visual range, synchronously with the first event signal; asecond laser source for radiating a second pulse laser beam having awavelength in the infrared or visual range, synchronously with thesecond event signal; an optical propagation system for changing anoptical axis of at least one of the first and second laser beams so asto make the first and second pulse laser beams propagate along a sameoptical axis; a non-linear optical component for generating a harmonicwave having a wavelength in an ultraviolet range, from the first andsecond pulse laser beams made to have the same optical axis by saidoptical propagation system; a converging optical system for convergingthe harmonic wave; and holder for holding a workpiece at a positionwhere the harmonic wave converged by said converging optical system isapplied.